A major challenge in mobile communication systems is to perform efficient data transmission at low terminal power consumption. Packet data transmission services in particular are characterized by extreme burstiness of the traffic volume that needs to be transferred over the radio interface with one user. Time intervals where large amounts of traffic need to be transmitted alternate with time intervals where there is only a very low volume traffic or no traffic at all.
This is relevant in the case of UMTS, and the invention will be described with reference to its use in a Wideband Code Division Multiple Access (WCDMA) wireless network, although it will be appreciated that the same principle may be applied in other networks.
If the physical link between a terminal and the network is maintained continuously, even when no user data actually needs to be transmitted, this leads to continuous power consumption of the terminal. On the other hand, maintenance of the physical link guarantees that, if new data arises, it can be transferred instantly without additional delay.
Maintenance of a channel is therefore favored. There are two main approaches to enable power saving in the terminal in this case.
One possibility is to use channel type switching. In this case, low-volume traffic is transmitted and received on common channels, while high-volume traffic is transmitted on dedicated channels. A potential problem with this approach is the need to perform a new link synchronization before each transmission on a common channel, and whenever transmission on a dedicated channel needs to be started again.
A second possibility is to perform discontinuous transmission and reception on dedicated channels. This possibility has different possible variants. One variant is referred to as “DPCCH Discontinuous transmission and reception (DPCCH DTRX)”.
The purpose of DPCCH DTRX is to switch transmission and reception on the Dedicated Physical Control Channel (DPCCH) off, into a “sleep mode” for a number of radio frames, in both the terminal and the network. At regular intervals, called Discontinuous Transmission periods (“DTX periods”) transmitters and receivers shall wake up and perform re-synchronization of the physical link. After re-synchronization it is possible to transmit a short message in either direction, which for instance could be a command to leave the DPCCH DTRX mode and return to normal continuous transmission. This command could be either a Layer 1 indicator sent on the DPCCH or a higher layer message carried on the Dedicated Physical Data Channel (DPDCH).
In general in DPCCH DTRX, the spreading codes are kept assigned all the time in both the transmitter and the receiver. However, when the sleep periods are rather long it might be feasible to perform sharing of the codes among multiple users for which DPCCH DTRX is active.
To enable DPCCH DTRX to be used successfully, re-synchronization of the physical link must be performed quickly, and in a power efficient way.